Pulsed laser beams include bursts or pulses of light, as implied by name, and have been used for photoalteration of materials, both inorganic and organic alike. Pulsed lasers, such as non-ultraviolet, ultra-short pulsed lasers with pulse durations measured in the nanoseconds to femtoseconds range, are often used in ophthalmic surgical procedures. Typically, a pulsed laser beam is focused onto a desired area of the material, such as the cornea, the capsular bag, or the lens of the eye, to photoalter the material in this area and, in some instances, the associated peripheral area. Examples of photoalteration of the material include, but are not necessarily limited to, chemical and physical alterations, chemical and physical breakdown, disintegration, ablation, photodisruption, vaporization, a the like.
One example of photoalteration using pulsed laser beams is the photodisruption (e.g., via laser induced optical breakdown) of a material. Localized photodisruptions can be placed at or below the surface of the material to produce high-precision material processing. For example, a micro-optics scanning system may be used to scan the pulsed laser beams to produce an incision in the material and to create a flap therefrom. The term “scan” or “scanning” refers to the movement of the focal point of a pulsed laser beam along a desired path. To create a flap of the material, the pulsed laser beam is typically scanned along a pre-determined region (e.g., within the material) in either a spiral pattern or a raster pattern. In general, these patterns are mechanically simple to implement (e.g., continuous) and to control for a given scan rate and for a desired focal point separation of the pulsed laser beam. These patterns are also generally efficient.
Despite these advantages, the spiral or raster pattern may impose limits on the creation of a flap (e.g., due to mechanical restrictions on the micro-optic based scanning system or the like). In general, faster scan rates are desirable, but existing laser scanning equipment may lag commanded laser positions along one axis or along both axes, thus, shortening or compressing one or more raster scan lines along another axis. For example, a circular scan area using a raster pattern may become elliptical with faster scan rates. In addition, faster scan rates may result in greater accelerations of the mass associated with the scanning system. These greater accelerations in turn complicate control accuracy. For example, greater accelerations have been observed while scanning the central region of a spiral pattern (e.g., as the spiral tightens). Greater accelerations have also been observed while scanning the periphery of a raster pattern (e.g., as the scanning changes direction with the raster pattern).
As such, systems and methods for scanning a pulsed laser beam that improve scanning control are desirable. More particularly, systems and methods for scanning a pulsed laser beam that reduce accelerations during scanning are desired. There is also a need for systems and methods for creating a flap with a pulsed laser beam operating at increased pulse repetition rates while maintaining or reducing the acceleration associated with scanning the pulsed laser beam.
Moreover, during the well-known LASIK (Laser-Assisted in Situ Keratomileusis) procedure, a pulsed laser is used to create a flap in the cornea to expose the corneal stroma, which is then photoablated with an excimer laser to correct vision problems such as myopia, hyperopia, astigmatism, and the like. Scanning an ultra-short wavelength pulsed laser beam over the cornea of a patient's eye creates contiguous small bubbles that expand to form a resection plane for the flap. The small gas bubbles are released when the corneal flap is lifted, so they do not interfere with the excimer laser's photoablation process. But, sometimes the gas bubbles diffuse through and accumulate, forming an opaque bubble layer (“OBL”) in the corneal flap bed. The OBL does not release when the flap is lifted, and a result, may interfere with flap creation and/or with the excimer laser's iris recognition capabilities, which are generally used for proper alignment and positioning of the laser beam. Since the OBL typically remains in the corneal flap bed for up to ten or more minutes, iris recognition errors may be as high as 10%, which may consequently affect the precision, accuracy, and effectiveness of the laser procedure and the surgical outcome. Therefore, it is also desirable to provide systems and methods for reducing and/or eliminating the formation of opaque bubble layers in the corneal flap bed.